1. Field of the Invention
The present invention relates to a DC brushless motor, and in particular to a DC brushless motor having a magnetic-field permanent magnet and to a control apparatus.
2. Description of the Related Art
Two types of DC brushless motors exist and are classified in terms of their rotor structures. The first type is the surface permanent magnet (SPM) in which a magnetic-field permanent magnet is mounted on the surface of the rotor yoke. The second type is the interior permanent magnet (IPM) in which a magnetic-field permanent magnet is embedded inside the rotor yoke.
Of these two types of DC brushless motors with different rotor structures, the surface permanent magnet (SPM) rotor type usually has a structure in which a stainless steel pipe is mounted in order to prevent damage to the magnetic-field permanent magnet by the centrifugal force from the high-speed operation of the motor. As a result of this structure, the surface permanent magnet (SPM) rotor type tends to exhibit an increase in the magnetic resistance of the magnetic circuit that is composed of a magnetic-field permanent magnet, a rotor yoke, and an armature. Moreover, because the motor is driven using a power supply that is chopped having frequency of in the range of a few kHz to 20 kHz, the flow of eddy currents in the motor tends to degrade the efficiency of the motor.
On the other hand, for the embedded rotor type, a slot into which a magnetic-field permanent magnet is inserted is pre-formed in the interior of the rotor yoke when the rotor yoke is stamped out during manufacture thereof. This eliminates the need for a stainless steel pipe, and thus substantially reduces the generation of eddy currents.
Depending on the configuration of the slot, an increase in leaked magnetic flux results, thus reducing the effective number of magnetic fluxes in the magnetic circuit and the effectiveness of the magnet. Therefore, it is necessary to provide measures to prevent these conditions.
Because of these structural differences, the two types of rotors differ significantly in their machine constants, resulting in substantially different motor output characteristics. It should be noted that, because the magnetic permeability of a permanent magnet is close to that of vacuum, the magnetic permeability of some parts of a permanent magnet is considered to be equivalent to that of air.
In the motor having the surface permanent magnet (SPM) type rotor structure, the d-axis inductance is equal to the q-axis inductance and the motor has linear structure of current and torque.
Because of their favorable acceleration and deceleration characteristics as well as torque and related control characteristics, these motor are widely used as servo motors.
Motors with the interior permanent magnet (IPM) rotor structure have the characteristic of the q-axis inductance being greater than the d-axis inductance (the counter salient pole property). This permits a maximum torque control using the reluctance torque in addition to the permanent magnet's active torque, thus producing high output and high efficiency characteristics.
Moreover, motors with the interior permanent magnet (IPM) rotor structure allow the regulation of the phases of the armature current with respect to the phases of the back electromotive force (b-emf). This enables the motors with the interior permanent magnet (IPM) rotor structure to run in an rpm range greater than the limits imposed by the DC link voltage of the inverter control apparatus and the back electromotive force (b-emf) of the motor. Therefore, motors with the interior permanent magnet (IPM) rotor structure are potentially highly applicable as motors that drive mobile units, for which compact size, high efficiency, and high operating range are some of the critical performance requirements.
Regions of motor operation can be divided into two classes: one in which the motor can be operated continuously, and one in which it can be operated only for a short time. These regions are based on the extent of rise in motor temperature. Without providing a detailed description of these regions, it suffices to say that they are called continuous rating and short-time rating, respectively. The current that can be supplied for a continuous-rating operation is called the continuous-rating current.
FIG. 20 shows the structure of a conventional interior permanent magnet (IPM) rotor. FIG. 21 shows the output characteristic that results when a conventional interior permanent magnet (IPM) rotor structure motor is operated using a continuous-rating current.
As shown in FIG. 21, the ratio for a conventional motor is 1.6 (9200/5600 rpm) between the maximum rpm that is achieved when the motor is operated so that its back electromotive force (b-emf) and armature current are in phase with each other (called "i.sub.d =0" control) and the rpm that is achieved when the motor is operated using field-weakening control.
Thus, field-weakening control permits a 1.6-fold increase in the range of rpm over which the motor can be operated continuously. Although not shown in the figure, this range can be further increased by supplying a current greater than the continuous-rating current to the armature.
It is possible to produce a compact, high-efficiency, wide-operating range motor by controlling a motor with a conventional interior permanent magnet (IPM) rotor using conventional techniques.
In a DC brushless motor with a conventional interior permanent magnet (IPM) rotor, however, the maximum increase in operating range that can be achieved using a continuous-rating current is limited to approximately 1.6-fold. Any further increase in operating range entails an increase in the armature current which precludes continuous operation. This can be a problem, for example, when the mobile unit that incorporates the motor needs to cruise at high speeds.
Battery-operated motors that drive mobile units such as electric cars and electric motor scooters must be compact and highly efficient and have a wide operating range. These requirements are germane to the control apparatus and the transmission mechanism as well as to the motor itself.
However, further increases in the operating speed range of a mobile unit using a conventional motor, within the range of continuous rating, require a multistage transmission device which makes the equipment bulkier.
Further, increasing the operating range for the motor itself requires a large armature current. An attendant increase in copper loss increases the heat dissipation, thus reducing the length of operable time for the motor. Conventionally, this requires devices to improve the cooling efficiency for the motor, thus resulting in an increase in motor size. On the control apparatus side, the current rating for the inverter switch device must be increased, and this also tends to increase the size of the control apparatus.